Relocate 2W
Instant (C)
Return target creature you control to its owner's hand, then terraform. (To terraform, sacrifice a land, search your library for a basic land card and put it onto the battlefield tapped, then shuffle your library.)

Rejuvenation Machine 3
Artifact (C)
Operate 2 (2: Attach an unattached creature you control to this artifact, or unattach a creature from it. Attached creatures can't attack or block. Operate only as a sorcery.)
T: Gain 1 life. If a creature is operating ~, gain 2 life instead.

Rainbow Elemental 5GG
Creature - Elemental (R)
Sunburst - ~ enters the battlefield with a +1/+1 counter on it for each color of mana spent to cast it.
Creatures you control enter the battlefield with a +1/+1 counter for each color of mana spent to cast them.
3/3

Two templating issue: Sunburst is an actual ability, not an ability word, and Rainbow Elemental should just be "Creatures you control have Sunburst (They enter the battlefield…)". there are more, but these are two big one.

Rejuvenation Machine's a nice design, though I can't help thinking something then a mere strictly better Marble Chalice would be more exciting even at common.

Jostle is a poster boy for my worries about the smallest {2/C} spells: it's a colorless spell with a very blue ability! And even if you don't get the defensive tapping, I can still think of a lot of uses for a Sorcery-speed untap effect (like that Robot Factory!).

Terraform is a great name. I'm mildly concerned about the shuffling, but Wizards used Cycling in Alara block, so that can't be so bad.

That's a somewhat large amount of tokens (especially 0/1s). We'll need more than Metalgrafter in the set to make it work.

Regarding Jostle, I had hoped that having to play it at sorcery speed for the non-blue cost would alleviate some of the color pie concerns, as there's no mistaking it's clearly better in a blue deck. I want the 2 cost to have some purpose, though, which is why it lets you double-dip on Operate cards in a non-blue deck. Jostle feels like less of a violation than Phyrexian mana, at least, but I would consider changing its effect.

Terraform is based on Mana Upheaval which Chah suggested earlier: Mana Upheaval (Sacrifice a land you control. If you do, reveal cards from the top of your library until you reveal a land card with a different name and put it onto the battlefield tapped. Put the remaining cards on the bottom of your library at random.). Terraform makes sure you get the exact color you want so that it works better with sunburst or heavy twobrid cards ({2/C}{2/C}{2/C}), but it does involve more shuffling. I could still go either way on this mechanic.

I'll keep working on common Operate cards, as I agree Rejuvenation Machine doesn't do much to get people excited about the mechanic.

I think the set would benefit from a slightly higher than average number of tokens, both to stall until you can fix your mana for sunburst, and to give you something to Operate your machines with. Maybe it would be good to include some support for using tokens in aggressive strategies too:
Twobrid Trumpet Blast {2/R}{2/R}
Sorcery
creatures you control get +2/+0 until end of turn

Terraform was really just a throwaway comment. I think the mechanic is interesting, although it has this "looks like downside" aspect to it (Timmies in particular hate sacrificing stuff), but the amount of sunburst makes it clear that it makes a good colorfixing mechanic that you can still play without (the twobrid cards generally will do better in traditional 1-2 color decks).

Good point about tokens. That would alleviate the "Hydropon" problem (as George Gone has put it) that appears at first to stem from Operate. It also makes it clear that you want to have more operate in the set, though, to limit the instances that your 0/1 turns out useless. You think Herbologist could make 1/1 sparolings? Green making 0/1s seems somewhat… inappropriate.

Every color but Black has had both tapping and untapping in recent years – Black has tapped, though not untapped – and given the previous cases of twobrid, I think Jostle is probably fine. Much less egregious than, say making it a twobrid Ice or Unsummon.

Really? Green gets to untap lands or its creatures. White taps creatures. Black taps its own creature as a cost. Red only untaps as part of stealing/multi attack affects, just like other colors may get tap/untap to achieve on-color effects (e.g. Clinging Mists, Veteran's Reflexes). All colors get the occasional flavor tap or untapping effect (Stabbing Pain, Village Bell-Ringer, Thundermaw Hellkite…). The occasional colorless stuff (Voltaic Key, Tumble Magnet, Clock of Omens) affects only artifact.

I entirely agree about Ice and Unsummon, but this still triggers my bleeding worries. I'd be fine with a {2/U}{2/U} Ghostform, Peel from Reality or even Downpour because I feel most colors will have more interesting options at CMC4. Job Fair feels entirely fine, and would still be with the purity effect (turn it into a WW Raise the Alarm).

One might draw a parallel with the hybrid cards in RTR: you are more careful about what you put on a {U/W} (which any guild with either color can grab) than what you put on a {U/W}{U/W} (which will be significantly harder to play if your deck isn't UW or UWc).

I wouldn't want more than a cycle of {2/C} cards in the set, for the reasons you cite with hybrid cards. I still think Jostle's effect is fine, but it could be restricted to tapping/untapping artifacts if needed.